1. Field of the Invention
The present invention relates to a process for producing a surface-treated steel sheet or strip (hereinafter called "sheet") for welded cans suitable for packing and storing foods, beverages, and the like.
2. Description of the Related Art
The materials for containers for foods, beverages and the like include generally tin-coated steel sheets and tin-free steel sheets. In recent years, thinly tin-coated steel sheets having a subdued amount of tin-coating (including those having nickel coating as an underlayer for assuring weldability), nickel coated steel sheets, and the like have also come to be used for the purpose of lowering the material cost. However, the tin-free steel sheet is much superior to the tin plate, and also to the thinly tin-coated steel sheets and the nickel-plated steel sheets, when compared simply in view of cost. The tin-free steel sheet is also excellent in paint adhesion and under-paint-coating corrosion resistance. Because of these excellent properties, the demand for tin-free steel sheets is rising. However, the tin-free steel sheet has a serious disadvantage of low weldability, and working of the tin-free steel sheet into cans has hitherto been conducted in most cases, by an adhesion method, or by a welding method in which a coating layer of metallic chromium and chromium oxide on the steel surface is removed by grinding before the welding. Therefore, the improvement of the weldability of the tin-free steel sheet is of great industrial significance, and various attempts have been made to improve the weldability of the tin-free steel sheet. For example, tin coating is applied as an underlayer prior to the chrome plating to improve the weldability as disclosed by Japanese Patent Publications Hei 2-16397 and Sho 61-1518, and Japanese Laid-Open Patent Applications Sho 56-127776 and Sho 56-44793. These attempts are all directed to the improvement of insulation properties of a chrome-coating and a chromium hydrate-oxide coating to provide improved weldability. In these improvements, however, a larger amount of tin-coating as the underlayer is required for decreasing contact resistance, which sacrifices the inherent desirable properties of a tin-free steel sheet in return for improvement of weldability similarly as in the case of thinly tinned steel sheets. Thus in all of the above attempts, the inherent desirable properties of tin-free steel sheets, such as metallic surface luster, high paint adhesion, and under-paint-coating corrosion resistance are lost even though the weldability of the tin-free sheet is improved. Therefore, the surface-treated steel sheets obtained by the above-described prior arts are not always satisfactory for the cans.
In order to solve the problems of the prior arts the inventors of the present invention developed a process for producing surface-treated steel sheet as disclosed in Japanese Laid-Open Patent Application Hei 2-282498 in which granular tin coating is applied on the steel sheet to impart weldability to the steel sheet without impairing the inherent desirable properties of tin-free steel sheets, namely metallic surface luster, high paint adhesion, and under-paint coating corrosion resistance.
The above disclosed process for producing a surface-treated steel sheet superior in weldability and paint adhesiveness comprises degreasing and acid-pickling a steel sheet; tin-plating the steel sheet in an acidic tin plating bath containing no conventional brightener under plating conditions: a bivalent tin concentration of 2-20 g/l, an acid concentration of 10-50 g/l (in terms of sulfuric acid), a plating current density of 2-15 A/dm.sup.2, and a plating amount of 20-200 mg/m.sup.2 so as to give tin coating deposits in a granular shape having a diameter of 0.2-1.5 .mu.m and subsequently coating the tin-plated steel sheet with metallic chromium in an amount of 30-150 mg/m.sup.2, and chromium oxide in an amount of 2-40 mg/m.sup.2 in terms of chromium by a conventional chrome plating process, or a conventional chrome plating and chromate treatment process.
This process provides a surface-treated steel sheet having weldability without impairing inherent superior properties of the tin-free steel sheet.
However, the above process of granular tin coating as the underlayer does not give sufficient adhesion of the granular tin deposits, so that the granular tin deposits are liable to fall off, after plating, during passing through a water-washing and other steps prior to the chrome plating due to slippage between a steel strip and rolls, or other causes during a continuous plating process of the steel strip. Accordingly, even if the amount of tin coating is controlled successfully in the tin-plating step, the amount of the granular tin coating will vary in the chrome plating step, which causes large variation of imparted weldability, and renders the quality control difficult.